The present invention relates to lasers and more particularly to electric discharge gas lasers (EDL) wherein an electric discharge in a gaseous medium produces a population inversion of energy states in the medium sufficient to support the laser action.
The present invention is an improvement over that shown and described in U.S. patent application, Ser. No. 650,309 filed Jan. 19, 1976 by Jacob L. Zar and Robert E. Serris, and assigned to the same Assignee as this patent application.
A laser beam is a beam of coherent electromagnetic radiation which by virtue of its coherence is highly directional and so the laser beam can be projected great distances with little spreading of the beam. Because the laser beam possesses space coherence, it can be focused to form a small spot. Hence, enormous power densities can be obtained.
An electron beam laser is described in U.S. Pat. No. 3,702,973, issued Nov. 14, 1972 entitled "Laser or Ozone Generator in Which a Broad Electron Beam with a Sustainer Field Produce a Large Area Uniform Discharge" to Daugherty et al. This patent describes a structure and method for operating a relatively large volume, high pressure gas electric discharge gas dynamic laser in which the medium contains CO.sub.2. A spacially uniform controlled electric discharge is produced in the working region by introducing ionizing radiation (a broad electron beam) into the laser optical cavity through a wall of the cavity to produce a substantially uniform predetermined density of secondary electrons in the gaseous medium by ionizing the medium. At the same time there is provided a sustainer electric field which is uniform throughout the working region of the laser and which provides a predetermined electron temperature which is calculated to increase the average energy of secondary electrons in the working region without substantially increasing the predetermined electron density in the region. This patent describes a method and structure for producing a uniform controlled discharge in a gaseous medium in a relatively large volume at relatively high pressure. The sustainer field direction, the laser beam direction and the gas flow direction may be mutually orthogonal.
In operation, the ionizing electron beam is generated outside the laser cavity by an E-beam generator and there is a broad area uniform beam of sufficient cross section dimension to cover the relatively large working region of the laser. A suitable structure for generating such a broad area uniform electron beam is described in U.S. Pat. No. 3,749,967 which issued July 31, 1973 entitled "Electron Beam Discharge Device" by Douglas-Hamilton et al. The beam is transmitted into the laser cavity through an electron window and into the working region bounded by the sustainer field. A portion of the laser optical cavity is included in that sustainer field and in the optical cavity.
In the high power electron discharge such as described in the above-mentioned U.S. Pat. No. 3,702,973, the output laser power is approximately proportional to the input power to the sustainer section. The sustainer section includes an anode and a cathode with the gaseous working region in between and so the working region of the laser is defined by this anode and cathode. It is the discharge between the anode and the cathode, uniformly maintained, that pumps the laser and so provides the inverted population of energy states necessary for laser action. Since the laser output power is proportional to the input power to the sustainer, the output power can be controlled by controlling the sustainer voltage. This technique has been effective for gas lasers of smaller size. However, it is not as effective for lasers of larger size, particularly where the laser output power must be changed rapidly. For relatively large electron beam lasers, the density of the electron beam projected into the working region between the sustainer electrodes is controlled while the sustainer voltage is held constant. Thus, the sustainer current is varied to vary output power of the laser. This, in turn, depends upon the ion concentration produced in the working region by the ionizing electron beam.
The ionizing electron beam is produced by the E-beam system which is an external electron accelerating device that generates a broad area electron beam which is projected through an electron window into the working region of the laser. In the E-beam device, electrons emitted by a cathode are accelerated by anodes and so the energy of the electron entering the working region of the laser is determined by the accelerating anode voltage. Usually, the accelerating anode voltage is maintained constant and the voltage on a controlled grid located between the accelerating anodes and the cathode is varied. This control grid controls the density of the electron beam from the device that is projected into the sustainer working region of the laser. Very abrupt changes in the laser output power can be achieved by abruptly changing the voltage on this control grid in the E-beam system. Thus, the E-beam device and the sustainer device operate in conjunction in a fashion similar to a triode or a tetrode vacuum tube to control the output power of the laser, that output power being controlled by a grid potential in the E-beam device.
Heretofore, an electron beam CO.sub.2 laser constructed and operated as described above and including an E-beam device and a sustainer has included a null-type feedback control system. The feedback control system detects or senses the current in the electron beam and compares that current with a standard generated by the operator, producing a control signal that reflects the difference. The control signal is applied to the E-beam device control grid. That feedback system, intended particularly to compensate for variations which might result from such things as changes in power line voltage, drifts in component values in the power supply or other factors that could affect the amplitude of the E-beam current. It was not completely effective to correct perturbations in the laser beam output and so, it was less effective than desirable where the output laser beam must be maintained steady and substantially free of perturbations and where the beam power must be changed abruptly as when the beam is pulsed.
In the feedback control system of the aforementioned Zar et al patent application, there is provided a control system for a laser using a feedback loop including an optical system and an electrical system. The combined optical and electrical systems that make up the feedback has particular use in the control of an electron beam laser.
The optical system utilizes mirrors to periodically intercept a laser beam and directs the intercepted radiation through optical attenuators to a radiation detector which produces an electrical signal representative of the power of the periodically intercepted beam and consisting of a series of pulses at the rate of interception of the laser beam. The successive pulses are combined by a conditioning circuit which produces a substantially steady signal level that is proportional to the power of the laser beam. That signal level is compared with a standard signal level controlled by the operator producing a difference signal that is used to control the E-beam device. The difference signal may be amplified and applied directly to the grid of the E-beam device, or as in one embodiment described herein, the difference signal is transmitted by a telemetry transmitter to a telemetry receiver at the location of the E-beam device where the received difference signal is extracted, amplified and applied to control the grid of the E-beam.
The output laser beam is periodically sampled by reflective mirror portions on a rotating wheel which must be disposed at an angle to the laser beam. Because the power reflected by the mirrors is higher than desired, the reflected output beam radiation is first attenuated several orders of magnitude in the optical system and then focused on the radiation detector that produces a signal representative of the power of the reflected radiation.
In one embodiment, there is also included in the signal conditioning circuit that conditions the signal from the radiation detector, means for averaging the electrical signal pulses from the detector. More particularly, the pulse train from the detector is electrically clamped at a level depending on the energy of the successive pulses and then full wave rectified with reference to that level and smoothed to produce a substantially steady output signal, the level of which is indicative of the power of the output laser beam and is substantially free of power line frequency harmonics that appear in the output laser beam. This steady signal level can then be compared with the standard signal level controlled by the operator to produce the difference signal that controls the E-beam device.
In order to make the feedback control system more responsive to abrupt changes in the laser output beam power level, as when the beam is intentionally pulsed, a signal derived from the E-beam device and representative of the E-beam current is differentiated and added to the averaged signal and then the sum of these two is compared with the standard controlled by the operator. The purpose of adding the differential of the E-beam current to the averaged signal derived from the output laser beam is to improve the transient response of the control system. It should be noted that the averaging process is inherently slower than the detector, and while the averaged signal is quite effective to control the laser during substantially steady operation, it is too slow to respond to abrupt changes in the laser output power as when the laser is pulsed. On the other hand, the E-beam device, and more particularly the E-beam current, directly turns the laser beam on and off and this current may even slightly lead the laser beam. Hence, the sum of the differential of the E-beam current and the control signal stabilizes the laser power control circuit and in effect permits a fast response where it is needed.
As may now be seen from the preceding discussion, it is difficult to sample high-power laser beams without markedly affecting the beam power, or indeed totally interrupting it. The use of rotating mirrors as disclosed in the aforementioned Zar et al patent application is subject to many disadvantages. Thus, typically, more energy than is desired is necessarily reflected out of the laser beam; the mirrors must generally be of a considerable size which not only controls the amount of energy reflected, but causes interruption of an appreciable portion of the laser beam; the energy reflected is highly dependent on the cleanliness of the mirror surfaces, surface films and dust, all of which tend to vary in time; and the choice of location of the means for measuring the energy reflected by a mirror is severely limited.